Humidity, dew point, frost point, temperature and barometric pressure measurements play a critical role in various scientific and industrial settings, including HVAC systems, clean rooms, glove boxes, industrial processes, pharmaceutical laboratories, emissions testing, greenhouse monitoring, weather stations, as a component in precision dew point and RH generators, environmental chambers, octane measurement, shipping containers, and ground transport of humidity-sensitive foodstuffs. These industrial applications require moisture and water vapor measurement over a very wide range of concentrations, which can vary from as much as 1% to less than 1 part per million by volume, and over a large range of ambient temperatures, from as little as −100° C. to as much as 225° C. Various techniques have been employed to measure these fundamental properties of water vapor and derived moisture measurement units.
One technique used to measure humidity, dew point, frost point, temperature or barometric pressure is referred to as a “chilled mirror hygrometer.” Chilled mirror hygrometers have been widely used in commercial applications since the 1960s. A schematic of a conventional chilled mirror hygrometer optical configuration is shown in FIG. 13. In a conventional chilled mirror hygrometer, a polished mirror surface is cooled through the use of a thermoelectric cooler, a Peltier stack, or through cryogenic means. The mirror surface is typically plated with a reflective and conductive material such as gold, platinum, rhodium, nickel, or chrome. A precision thermometer, such as a platinum resistance thermometer (PRT) contacts or is bonded to the polished mirror. A high intensity light-emitting diode (LED) illuminates the polished mirror surface and the amount of light that is reflected from the surface of the mirror is detected with a photo-transistor or optical detector.
Chilled mirror hygrometers measure humidity, dew point, frost point, temperature or barometric pressure by observing the temperature at which dew or frost forms on the surface of the polished mirror. When the temperature of the mirror surface is reduced below the dew or frost point by the thermoelectric cooler, moisture forms as dew or frost on the polished mirror. Because of the presence of moisture, the light hitting the polished mirror is scattered and the amount of light reflected by the mirror onto the photo-transistor or optical detector is therefore reduced. The photo-transistor or optical detector observes the reduction in light being reflected towards it by the polished mirror and the precision thermometer provides the temperature at which the reduction in reflected light occurred.
Some chilled mirror hygrometers seek to establish a condensate equilibrium at the dew or frost point temperature at the polished mirror surface. As dew or frost forms on the polished mirror surface, the light being observed by the photo-transistor or optical detector is reduced. In some chilled mirror hygrometers, this causes the cooling device to begin raising the temperature of the polished mirror surface either through a thermo optical servo system or by means of a digital controller. As the temperature on the mirror rises, the dew or frost on the polished mirror surface eventually begins to disappear, which causes the amount of light observed by the photo-transistor or optical detector to increase. This increase in observable light then causes the cooling device to begin lowering the temperature, again, either through a thermo optical servo system or by means of a digital controller. The changes in temperature and observable light during this cycle are minute and, in effect, establish a condensate equilibrium at the dew or frost point temperature at the polished mirror surface.
The precision of chilled mirrors are affected, over time, by the accumulation of dirt and other contaminants on the polished mirror surface. Chilled mirror hygrometers require access to the ambient air or an air sample in order to take measurements. These air samples often times contain dirt and other contaminants that are deposited on the chilled mirror surface. Maintaining a condensate equilibrium at the dew or frost point temperature on the polished mirror surface further encourages the accumulation of contaminants because dirt and other contaminants accumulate more readily in the presence of moisture. One solution to the accumulation of dirt is to periodically open the chilled mirror hygrometer and mechanically clean the polished mirror surface. However, this method may scratch the polished mirror surface, which causes unwanted nucleation sites to appear on the mirror surface. Furthermore, this method will not prevent accumulated dirt from affecting the measurements of the hygrometer during operation.
A variety of calibration procedures may also be used to continually recalibrate a chilled mirror hygrometer to account for the presence and buildup of dirt and other contaminants on the polished mirror. One re-calibration technique is referred to as an “automatic balancing control” or “ABC” cycle. Using this technique, the chilled mirror hygrometer is programmed to heat the polished mirror surface to a temperature well in excess of the dew point to ensure that the mirror will be dry. The hygrometer then measures the loss in reflectivity of the polished mirror surface that is attributable to contaminants, alone, and in the absence of any moisture by comparing the reflectivity of the dry, but dirty, mirror against a baseline measurement of light directly from an LED to an optical sensing circuitry. Based on this reading, the optical sensing circuitry is automatically adjusted to compensate for any loss in reflectivity that is caused by contamination on the mirror surface.
The “Programmable Automatic Contaminant Error Reduction” or “PACER” cycle also adjusts optical sensing circuitry to account for the presence of contaminants on the polished mirror surface. Unlike the ABC cycle, the PACER technique begins by first reducing the temperature of the chilled mirror below the dew point, allowing water to condense on the surface of the mirror. The condensed water then forms into puddles on the polished mirror surface. These puddles cause soluble materials to dissolve. The PACER circuit then causes the mirror to be rapidly heated to a temperature well in excess of the dew point in order to ensure that the mirror rapidly dries. The puddles that formed on the polished mirror surface rapidly shrink as the water evaporates, increasing the concentration of contaminants in the shrinking puddle and leaving behind a clean mirror surface. Eventually the puddles become so concentrated that solute begins to precipitate out in polycrystalline clusters, which remain once the mirror is completely dry. This process redistributes the salts and other contaminants so as to concentrate them in spots around the mirror as opposed to contaminating the entire mirror surface. Even on severely contaminated mirrors, the resulting reduction in reflectance is only about 15-20%. Once the polished mirror is dry, the optical sensing circuitry can also determine the resulting reduction in reflectivity resulting from the redistributed contamination and adjust its calibration accordingly.
However, neither the ABC nor the PACER cycle will completely remove the need to periodically clean the polished mirror surface. Both of these methods merely delay the need for mechanically cleaning the polished mirror, thereby reducing wear and tear and scratching of the mirror surface over the life of the chilled mirror hygrometer. Once the polished mirror becomes too scratched or damaged from repeated cleanings, either the mirror or the hygrometer must be replaced.
The LED brightness and the sensitivity of the photo-transistor or optical detector can also be directly affected by several factors present within the air sample, itself, such as dirt, temperature, and moisture levels. To compensate for these additional sources of potential imprecision, chilled mirror hygrometers typically contain two pairs of LEDs and photo-transistors or optical detectors, as shown in FIG. 13. One pair of optical components is used to measure the presence of dew or frost on the polished mirror surface. The second pair of optical components is used as a reference or control to measure the effect of the air sample, itself, on the optical components in order to correct for fluctuations in the light caused by the air sample and not the presence of moisture on the polished mirror surface. However, the need for two sets of optical components requires a significant amount of space and presents challenges in small chilled mirror hygrometers. In small chilled mirror hygrometers, designers may forgo the additional precision afforded by including a control LED in order to maximize space. The additional optical components also increase the manufacturing costs for chilled mirror hygrometers.
Thus there is a need for a chilled mirror hygrometer that reduces the number of optical components required to operate a chilled mirror hygrometer. Likewise, there is a need to reduce the amount and frequency of re-calibration required to provide precise and accurate readings from a chilled mirror hygrometer.